Summary of findings 1. Comparison of strategies for enhancing the implementation of school‐based policies or practices targeting risk factors for chronic disease.

Outcome	Anticipated absolute effects		Relative effect (95% CI)	№ of participants (studies)	Certainty of the evidence	What happens
	Risk with control	Risk with experimental
Implementation of policies, practices or programmes that promote student health in schools	Mean PA practices achieved 0.5 (SD 0.7)a	SMD of 1.04 is equivalent to an increase in the implementation of 0.73 of a school chronic disease prevention policy or practice	SMD 1.04 SD higher (0.74 higher to 1.34 higher)	897 schoolsb (22 RCTs)	⊕⊕⊖⊖ Lowc	Implementation strategies may result in a large increase in the implementation of interventions in schools.
Student diet	Dietary diversity score (range 0–9) mean 4.54 (SD 1.22)d	SMD of 0.08 is equivalent to an improvement in dietary diversity score of 0.10 units	SMD 0.08 SD higher (0.02 higher to 0.15 higher)	16,649 participants (11 RCTs)	⊕⊕⊖⊖ Lowe	Implementation strategies may result in slight improvements in student nutrition outcomes.
Student physical activity	Mean steps/day 2556.85 (SD 557.27)f	SMD of 0.09 is equivalent to an improvement of 50 steps/day more	SMD 0.09 SD higher (0.02 lower to 0.19 higher)	16,389 participants (9 RCTs)	⊕⊕⊖⊖ Lowg	Implementation strategies may result in slight improvements in student physical activity outcomes.
Student obesity	Mean BMI 19.1 (SD 3.7)h	SMD of −0.02 is equivalent to a reduction of BMI by 0.074 points	SMD 0.02 SD lower (0.05 lower to 0.02 higher)	18,618 participants (8 RCTs)	⊕⊕⊕⊖ Moderatei	Implementation strategies probably result in little to no difference in measures of student obesity.
Student tobacco use	See comments	See comments	SMD 0.03 SD lower (0.23 lower to 0.18 higher)	3635 participants (3 RCTs)	⊕⊖⊖⊖ Very lowj	We are very uncertain about the effect of implementation strategies on tobacco use outcomes.
Adverse events	See comment	See comment	—	(3 RCTs)	⊕⊕⊖⊖ Lowk	Interventions had little to no impact of on adverse events.
Cost‐effectiveness	See comment	See comment	—	(4 RCTs)	⊕⊖⊖⊖ Very lowl	We are uncertain whether strategies to improve the implementation of school‐based policies, practices or programmes targeting risk factors for chronic disease are cost‐effective.
BMI: body mass index; PA: physical activity; RCT: randomised controlled trial; SD: standard deviation; SMD: standardised mean difference.
GRADE Working Group grades of evidence High certainty: we are very confident that the true effect lies close to that of the estimate of the effect. Moderate certainty: we are moderately confident in the effect estimate: the true effect is likely to be close to the estimate of the effect, but there is a possibility that it is substantially different. Low certainty: our confidence in the effect estimate is limited: the true effect may be substantially different from the estimate of the effect. Very low certainty: we have very little confidence in the effect estimate: the true effect is likely to be substantially different from the estimate of effect.

^aBaseline estimates and SDs used from Sutherland 2020.
^bNumber of school reported rather than participants as implementation data were not reported at the participant level and may have included school, class, individual or some other level data.
^cDowngraded one level for unclear/high risk of bias and one level for substantial heterogeneity (I² = 81%; 50% to 90% considered substantial heterogeneity).
^cBaseline estimates and SD used from de Villiers 2015.
^dDowngraded one level for unclear/high risk of bias and one level for substantial heterogeneity (I² = 63%; 50% to 90% considered substantial heterogeneity).
^eBaseline estimates and SD used from Nathan 2020.
^fDowngraded one level for substantial heterogeneity (I² = 83%; 50% to 90% considered substantial heterogeneity) and one level for imprecision as 95% CI included both benefits and harms.
^gBaseline estimates and SD used from Naylor 2006.
^hDowngraded one level for imprecision as 95% CI included both benefits and harms.
ⁱDowngraded one level for unclear/high risk of bias, one level for substantial heterogeneity (I² = 81%; 50% to 90% considered high heterogeneity), and one level for imprecision as 95% CI include both benefits and harms.
^jDowngraded one level for unclear/high risk of bias and one level for small number of studies reporting adverse outcomes.
^kDowngraded one level for unclear/high risk of bias, one level for indirectness given the small number of studies providing assessment on cost‐effectiveness, and one level for imprecision given the small number of schools in total.